Increased Training Intensity Induces Proper Membrane Localization of Actin Remodeling Proteins in the Hippocampus Preventing Cognitive Deficits: Implications for Fragile X Syndrome.

Behavioral intervention therapy has proven beneficial in the treatment of autism and intellectual disabilities (ID), raising the possibility of certain changes in molecular mechanisms activated by these interventions that may promote learning. Fragile X syndrome (FXS) is a neurodevelopmental disorder characterized by autistic features and intellectual disability and can serve as a model to examine mechanisms that promote learning. FXS results from mutations in the fragile X mental retardation 1 gene (Fmr1) that prevents expression of the Fmr1 protein (FMRP), a messenger RNA (mRNA) translation regulator at synapses. Among many other functions, FMRP organizes a complex with the actin cytoskeleton-regulating small Rho GTPase Rac1. As in humans, Fmr1 KO mice lacking FMRP display autistic-like behaviors and deformities of actin-rich synaptic structures in addition to impaired hippocampal learning and synaptic plasticity. These features have been previously linked to proper function of actin remodeling proteins that includes Rac1. An important step in Rac1 activation and function is its translocation to the membrane, where it can influence synaptic actin cytoskeleton remodeling during hippocampus-dependent learning. Herein, we report that Fmr1 KO mouse hippocampus exhibits increased levels of membrane-bound Rac1, which may prevent proper learning-induced synaptic changes. We also determine that increasing training intensity during fear conditioning (FC) training restores contextual memory in Fmr1 KO mice and reduces membrane-bound Rac1 in Fmr1 KO hippocampus. Increased training intensity also results in normalized long-term potentiation in hippocampal slices taken from Fmr1 KO mice. These results point to interventional treatments providing new therapeutic options for FXS-related cognitive dysfunction.